To improve the user capacity of system, a low frequency reuse actor is often adopted in the prior wireless communication system, namely, all cells share a same time frequency resource, while the inter-cell interference (ICI) caused by low frequency reuse factor is an important factor in restricting the downlink capacity. For an mobile station (MS) located in cell-edge (i.e. the area between neighboring cells), while receiving a useful signal from the base to which the MS belongs, the MS still receives signals via the same time/frequency from other Base stations (BS), which forms a interference to the MS.
In the prior wireless communication system, there exists the following several solutions for the above problem:
1. Fractional Frequency Reuse
On the basis of the solution of Fractional Frequency Reuse, the user in the central area of the cell is permitted to work with all available sub-channels, while the user in the cell-edge are is only permitted to work with a portion of all the available sub-channels, wherein the assignment of the sub-channels should ensure that different sub-channel group is assigned to the edge area of neighboring cells. The technology of fractional frequency reuse maximizes the spectral efficiency of the central area of cells and improves signal quality of users in cell-edge.
2. Macro-Diversity
On the basis of the solution of macro-diversity, for the MS moving over to cell-edge, the BS to which the MS belongs to and the BS to which the MS is moving over send the same signal to the MS via the same time/frequency (slot and sub-carrier). Thus, ICI of neighboring cells is restricted, and diversity gain can be obtained for the received signal of the MS due to the independence of transmission channel among the BSs.
For the improvement of performance of users in the cell-edge without loss (or even with improvement) of total throughput of cells, applicant of the present intention propose a technology of multiple BS MIMO. As a portion of such technology of multiple BS MIMO, applicant of the present invention submit a technical proposal to broadband wireless access working group of IEEE802.16 on May 5, 2008, and the number of the proposal is IEEE C802.16m-08/423, of which the name is “Multiple Base Station MIMO”. The proposal is incorporated entirely herein by reference.
The basic concept of such technology of multiple BS MIMO is the collaborative MIMO transmission and receiving between multiple BSs and one or more MSs via the sane wireless resource. Such technology is with basic property of two aspects:
1) With the coordination of BS, multiple BSs jointly serve for the same MS via the same time frequency.
2) each BS may serve multiple MSs via the same time/frequency.
Multiple BS MIMO includes basic operations of the following three aspects:
1) each BS acquires the channel information of all MSs for which it serves. Channel information may be acquired in the following different fashions:
i. for TDD Downlink, estimation of Uplink is applicable to Downlink transmission.
ii. for FDD Downlink, channel information may be acquired by feedback of MS.
2) Information exchange is necessary for coordination of BS. And the manners to implement information exchange may be as follows:
i. whole information exchange: every BS needs to share all the channel information between the BS and MSs that the BS possibly may serve for, for instance short-time channel matrix.
ii. Partial information exchange: every BS merely needs to share portion of the channel information between the BS and MSs that the BS possibly may serve for, for instance, long-time channel quality indicator (CQI), Received Signal Strength Indication (RSSI) and measurement of preamble of other downlink or covariance channel matrix.
iii. Schedule information (for instance time, frequency, transmission format and so on) may have to be shared between BSs.
3) base on the information acquired by the procedure 1), 2), multiple BS transmission is conducted via the Downlink of MSs served jointly.
Previously, on Aug. 29, 2007, applicant of the present invention submitted a technical proposal, numbered IEEE C802.16m-07/16, to the broadband wireless access working group of IEEE802.16, of which the title is “Collaborative MIMO Based on Multiple Base Station Coordination”. The proposal discloses a relatively specific technical solution of multiple BS MIMO, briefly referred to as Co-MIMO. The proposal is incorporated entirely herein by reference. The proposal discloses an exemplary Co-MIMO system architecture: multiple cooperative BSs are connected to a scheduling device via Backbone network, and each BS comprises a device for generating pre-coding matrix so as to compute the pre coding matrix thereof to perform the Space Division Multiple Access (SDMA). Following steps will be performed on BS side:
1) each BS reports some channel information, between the MS and itself, to the scheduling device via Backbone network, for example Received Signal Strength Indication (RSSI) or channel quality indicator (CQI).
2) the scheduling device determines serving relationship between every BS and every MS, and send the schedule decision to every cooperative BS. A device for dispatching user data service dispatches user data stream to the corresponding BS according to the scheduling decision. Wherein, di represents the user data stream to be transmitted by the base station BSi.
3) According to the scheduling decision, the base station BSi estimates the channel status information (CSI) Hi between itself and the MS served by itself. And then, the device for generating pre-coding matrix of the base station BSi independently compute the pre-coding matrix Wi thereof according to Hi, thereby carrying out beamforming or the technology of multiple user MIMO.
Herein, all those MIMO technologies capable of implementing the following two points are commonly referred to as multiple BS MIMO technology:
1) by the coordination of BS, multiple BSs may jointly serve the same MS via the same time/frequency.
2) every BS may serve multiple MSs via the same time/frequency.
Currently, more investigation should go on with the problem that which BS Group should be coordinated to conduct the collaborative MIMO transmission in the multiple BS MIMO.